ENVR 403 Introduction to Environmental Chemistry Philip C. Singer Department of Environmental Sciences and Engineering April 2009

Topics 1. Chemistry of Natural Waters 2. Purification of Water

1. Chemistry of Natural Waters Redox Equilibria -speciation -dissolved oxygen resources Acid-Base Equilibria -speciation -the carbonate system and alkalinity Solubility Equilibria -metal solubility -aqueous lead (Pb) solubility

pH and pE as Master Variables NO H + + 8e - = NH H 2 OK red pE = -1/8 (pK red ) + 1/8 (Log [NO 3 - ] [H + ] 10 /[NH 4 + ]) pE o = -1/n (Log K red ) pE = pE o + 1/n Log [Ox]/[Red]) Analogy to Nernst Equation E H = E o H RT/nF (Log [Ox]/[Red]) H 2 CO 3 = HCO H + K a1 pH = pK a1 + Log [HCO 3 - ]/[H 2 CO 3 ]

After M. Benjamin, 2002

Dissolved Oxygen Depletion (Oxygen Demand) Carbonaceous oxygen demand (CBOD) CH 2 O + O 2 = CO 2 + H 2 O C 6 H 5 OH + 7O 2 = 6CO 2 + 3H 2 O Nitrogenous oxygen demand (NOD) NH O 2 = NO H 2 O + 2H + Chemical oxygen demand (COD) 3CH 2 O + 2Cr 2 O H + = 3CO 2 + 4Cr H 2 O

d[C]/dt = -k L [CBOD] - k N [NOD] + k H (Cs-C) where C = dissolved oxygen concentration CBOD = carbonaceous oxygen demand NOD = nitrogenous oxygen demand Cs = dissolved oxygen saturation concentration Dissolved oxygen sag curve

Nutrient Enrichment of Surface Waters and Eutrophication Photosynthesis CO 2 + NO PO H 2 O  Algae biomass + O 2 (C 106 H 263 O 110 N 16 P) Respiration Algal biomass + O 2  CO 2 + NO PO H 2 O

Electroneutrality Equation (Charge Balance) for Natural waters Major Cations: Na +, K +, Ca 2+, Mg 2+ Major Anions: Cl -, SO 4 2-, NO 3 -, HCO 3 -, CO 3 2- Charge Balance [Na + ] + [K + ] + 2[Ca 2+ ] + 2[Mg 2+ ] + [H + ] = [Cl - ]+ 2[SO 4 2- ] + [NO 3 - ] + [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] Cb-Ca = Acid neutralizing Capacity = Alkalinity = [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] – [H + ]

What is the pH of pure rainwater? Pure rainwater contains only dissolved CO 2 in equilibrium with the atmosphere. H 2 CO 3 = K H P CO2 ~ M What is the pH of a M solution of H 2 CO 3 ? Cb-Ca = Alkalinity = 0 [H + ] = [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] = K 1 [H+]C/D + 2 K 1 K 2 C/D where D = [H + ] 2 + K 1 [H + ] + K 1 K 2 Answer: pH 5.65

If a body of water has a pH of 7.5 and an alkalinity of 2x10 -3 equivalents/L, what is the equilibrium pH of the water if eq./L of acid is added to the water? Use initial pH and given alkalinity to find total carbonate concentration C C = (Alkalinity + [H + ] - [OH - ]) /((K 1 [H + ]/D) + 2K 1 K 2 /D) = 2.12x10 -3 M Added acid consumes alkalinity New alkalinity = 2x10 -3 – = eq/L Find new pH from alkalinity expression Alkalinity = [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] – [H + ] = ((K 1 [H + ]/D) + 2K 1 K 2 /D) 2.12 x [OH - ] – [H + ] Find pH = 6.35

After M. Benjamin, 2002

ENVR 419 Chemical Equilibria of Natural Waters Fall :00-12:15 Tu/Th

2. Purification of Water Drinking Water Wastewater Contaminated Groundwater

Sources of Drinking Water Fresh Water Ground Water Surface Water -Rivers and streams -Lakes and impoundments Brackish Water Reclaimed Wastewater? Importance of Watershed Protection

Potential Contaminants in Drinking Water Pathogenic microorganisms Bacteria (salmonella, cholera), viruses (Hepatitis A virus, poliovirus), protozoan cysts (Giardia, Cryptosporidium) Suspended particles (turbidity) Dissolved inorganic contaminants e.g. Fe, Mn, As, Cr, Cu, Pb, NO 3 -, ClO 4 - Dissolved organic contaminants e.g. taste and odor-causing organics, pesticides, pharmaceutically active compounds

Objectives of Drinking Water Treatment Removal of particulate material Removal of color (natural organic material, humic substances arising from vegetative decay) Removal of taste and odor-causing substances Removal of harmful contaminants - chemical and microbial Residual protection of water during distribution - protect against microbial recontamination, corrosion

Chemicals Used in Water Treatment Coagulants Aluminum, ferric salts Polymers Disinfectants Chlorine, chlorine dioxide, ozone (UV-Irradiation) Acids/Bases for pH adjustment Lime, caustic Sulfuric Acid Corrosion Control Phosphates, silicates Other Fluoride

Feed WaterFiltered Water (Permeate) Membrane Filtration u Thin barrier or film of material that allows certain substances to pass through while rejecting other substances.

Membrane Removal Functions Microfiltration (MF) – particles, bacteria, cysts Ultrafiltration (UF) – viruses and colloids Nanofiltration (NF) – viruses; natural organic matter; SOCs; hardness Reverse Osmosis (RO) – dissolved minerals (desalination)

Membrane Filtration Comparison Feed Water RO NF UF MF Particles, Giardia, Cryptosporidium Some Viruses DOC, Hardness Minerals Water

Objectives of Wastewater Treatment Removal of suspended solids Removal of oxygen-demanding material CBOD (e.g. CH 2 O), NOD (NH 3 ) Removal of nutrients that can promote algal growth, eutrophication Nitrogen, phosphorus Removal of toxic substances Disinfection

Metal Binding Agents in Detergents Dissolved calcium in hard waters precipitates when the pH is raised during normal laundering operations Dissolved calcium and magnesium in hard waters precipitates the cleansing agent in detergents Hence, detergents contain chelating agents that bind calcium Common chelating agents – polyphosphates, NTA, EDTA, citric acid, succinic acid

Groundwater Contamination NAPLs DNAPLs

Groundwater Remediation Pump and treat Use of surfactants In-situ biodegradation/bioremediation Reductive dehalogenation (zerovalent Fe o ) Isolation